Known optical power meters use a photodiode detector to produce an electrical current that is dependent upon the intensity of light incident upon the detector and an amplifier to produce a voltage proportional to the electrical current. Photodiode amplifiers are described extensively in the text book “Photodiode Amplifiers” authored by J. G. Graeme and published by McGraw-Hill, New York, 1996. Typically, the photodiode amplifier comprises an operational amplifier with a feedback resistor coupled to the inverting input port, the output voltage of the amplifier then being proportional to the photodiode current which is applied to the inverting input port of the operational amplifier. The largest measurable value is determined by the amplifier saturation characteristics, whereas the smallest measurable value is determined by the amplifier noise and the resistor noise; the optical dynamic range of such photodiode amplifier configuration typically is about 30 dB (i.e., 103). In contrast, photodiode detectors are capable of measuring optical power levels ranging from −110 dBm to +10 dBm.
In practice, therefore, the dynamic range of the light intensity may well exceed that of the amplifier. It is known to extend the effective dynamic range of such a photodiode amplifier by providing several feedback resistors in parallel, each with a different resistance value, and a gain selection switch for changing the amplifier gain by connecting different ones of the resistors into the feedback loop. Such an amplifier, having three gain settings, is shown in FIG. 1, labelled PRIOR ART. If the single-resistor photodiode amplifier has an optical dynamic range of about 30 dB (i.e., 103), the total optical dynamic range of such a three-gain range photodiode amplifier might be as much as 90 dB (i.e., 109).
A disadvantage of the amplifier shown in FIG. 1, and other known multi-range photodiode amplifiers, however, is that the operation of the gain selection switch causes glitches, signal loss and transient effects, which may result in invalid readings, or require readings to be interpolated where there are gaps in the readings coinciding with selection switch operation.
It is also known to improve the dynamic range of the basic photodiode amplifier by using a differential photodiode amplifier. Thus, in their U.S. Pat. No. 5,287,340, D. B. Chapman et al. describe a photodiode amplifier in which the photodiode cathode and anode are connected to respective inputs of a differential amplifier. The output voltage of the operational amplifier is proportional to the photodiode current. According to Chapman et al., this arrangement reduces shot noise and hence improves dynamic range Chapman et al. do not, however, disclose gain switching or address the attendant problems described above.